Natural gas has several advantages over other hydrocarbon fuels that are combusted in internal combustion engines. For example, natural gas is less expensive relative to other hydrocarbon fuels. Moreover, for example, natural gas burns cleaner during operation of the internal combustion engine relative to other hydrocarbon fuels. By burning cleaner, a reduced amount of combustion byproducts such as carbon monoxide, oxides of nitrogen or hydrocarbons are released into the environment during operation of the internal combustion engine. In addition, because lubricants of the internal combustion engine become contaminated with the combustion byproducts over a time period, the production of a reduced amount of combustion byproducts results in less contamination of the lubricants over the time period thereby increasing the useful life of the lubricants.
One type of internal combustion engine is a diesel engine. Diesel engines combust fuel by compressing a mixture of air and fuel to a point where the fuel is ignited by heat which results from such compression. When natural gas is used as a fuel in a diesel engine, the natural gas does not readily ignite as it is compressed. In order to overcome this problem, an ignition source is provided to ignite the natural gas. The ignition source may be provided by a spark plug similar to those used in spark ignition engines. However, in certain types of diesel engines (e.g. dual fuel engines), the ignition source is provided by injecting a stream of diesel pilot fuel, into a mixture of air and natural gas (or other gaseous fuel). As the mixture of air, natural gas, and diesel pilot fuel is compressed, the diesel pilot fuel ignites, which in turn ignites the natural gas.
When operating an engine with a mixture of natural gas and pilot diesel fuel, the presence of the pilot diesel fuel during combustion of the mixture results in generation of a certain amount of oxides of Nitrogen (NO.sub.x). This certain amount of NO.sub.x is greater than the amount of NO.sub.x produced when the engine is operated on natural gas alone. This is true since the composition (i.e. ratio of air to fuel) of the mixture varies depending on its proximity to the stream of diesel pilot fuel being injected into the combustion chamber. In particular, the mixture of natural gas and pilot diesel fuel near the stream of diesel pilot fuel is rich, while the mixture away from the stream of diesel pilot fuel is lean. Combustion of rich mixtures tend to produce a substantial quantity of NO.sub.x. (Note that combustion of lean mixtures tend to produce a substantially reduced quantity of NO.sub.x).
What is needed therefore is a dual fuel engine which generates a relatively small amount of NO.sub.x and other combustion byproducts during operation of the engine.